Electron microscope having a light metal layer on the interior walls to prevent x-ray generation



3,327,112 ELECTRON MICROSCOPE HAVING A LIGHT METAL LAYER ON THE June 20,1967 HIROSHI AKAHORI INTERIOR WALLS TO PREVENT X-RAY GENERATION FiledFeb. 25, 1964 lNvENToR Hmosm flxfixom Y: 1% m TO United States PatentELECTRON MICROSCOPE HAVING A LIGHT METAL LAYER ON THE INTERIOR WALLS TOPREVENT X-RAY GENERATION Hiroshi Akahori, Katsuta-shi, Japan, assignorto Hitachi, Ltd., Tokyo, Japan, a corporation of Japan Filed Feb. 25,1964, Ser. No. 347,276 9 Claims. (Cl. 250-495) This invention relates toan electron discharge device and more particularly to such a devicehaving a layermade of light metal disposed on the inner surface ofselective portions of its wall for preventing X-ray generation fromsecondary electrons.

As X-rays are highly harmful to the human body, electron devices such aselectron microscopes and electron machining apparatus must provide meansfor completely preventing passage of X-rays through the wall of thedevice.

For the purpose to prevent this X-ray leakage, in the prior art electrondischarge device, the walls of both an electron gun and an electronaccelerating means are made of heavy metal such as iron or copperprimarily to prevent outward leakage, and since outward leakageincreases with accelerating voltage the wall of the device has beenoften increased in thickness to meet such increase in outward leakage.

It has been found, however, from measurement that the outward leakageX-ray dose increases at a faster rate than does the accelerating voltageand the leakage X-ray spectrum comprises apparently a multiplicity ofspectra. It has been also found from investigation that, besides primaryX-rays which are produced by impingement of primary electrons upon meanssuch as an anode and a specimen arranged in the path of the primaryelectrons, secondary X-rays are generated at the wall of the device byimpingement of recoil and secondary electrons.

Since the dose and intensity of X-rays generated when an electronimpinges upon a surface of metal is approximately proportionate to the%th power of the atomic number of the metal in general, the quantitygenerated increases as the metal becomes heavier. Accordingly, ifheavier metal is used for the wall of the device, the quantity ofsecondary X-ray generated increases still more. As a result, in order toprevent completely the outward leakage of the secondary X-ray inaddition to the primary X-ray, the wall of the device must be soincreased in thickness that the device becomes disadvantageouslylargesized and costly.

As there is applied 'a high potential difference between the grid andthe wall of an electron gun, the inner surface of its wall is oftenglazed or chromium plated in order to eliminate pin-holes andirregularities which would cause discharge between the grid and thewall.

Since exceedingly great thickness is required for the wall of the-devicein order to prevent completely outward leakage of the primary andsecondary X-rays, the inner surface of the wall of the device must bepolished after cutting to glaze the surface. Further, since variousattachments such as an exhaust pipe must be mounted on the wall of thedevice, handling during machining is exceedingly troublesome. As aresult, the machining operation not only becomes complex but it is sodifficult to polish precisely the inner surface of the wall of thedevice that it can not be avoided that some irregularities and pin-holesremain in the inner surface of the wall of the device. On the otherhand, when the inner surface is chromium plated, as the plating layerinherently contains always some pinholes, the plated surface oftenrequires bufiing to remove such pin-holes, But since the wall of thedevice must be very thick to prevent outward leakage of the primary andsecondary X-rays completely and be provided with vari-.

ous attachments such as an exhaust pipe mounted thereon, it is verydifficult to finish precisely the inner surface of the wall by bufiing.Moreover, the chromium plating layer is so hard that it is verydiflicult to stop the pin-holes. Consequently it can not be avoided thatsome pin-holes remain and some irregularities are formed in the surfaceof the wall. As a result, these pin-holes and irregularities cause smalldischarges to be formed between the grid and the wall of the device.These small discharges effect a disturbance of the accelerating electronbeam and result in damage of the wall of the device. Moreover, electronsproduced in the small discharge impinges upon the wall of the deviceresulting in secondary X-ray radiation.

When the wall of the device is damaged by such small discharge, repairor replacement is required. If heavy metal is used as the material ofthe wall of the device, as done in the prior art, the wall of the devicemust be very thick to prevent completely outward leakage of secondaryX-rays, which results in very high cost and large expenditure inreplacement.

Thus the conventional electron device has many difficulties as mentionedabove. It is an object of the present invention to overcome suchdisadvantage inherent in prior art electron devices.

An important feature of the present invention is to prevent outwardleakage of X-rays by provision of a layer on the inner surface of thewall of the device which is made of light metal such as aluminium ormagnesium to prevent generation of X-rays without making the wall of thedevice and the layer for preventing X-ray generation unduly thick.

Another feature of the present invention is to reduce generation ofdischarge by provision of a cylinder for preventing X-ray generationbetween an electrode and the wall of an electron gun.

Still another feature of the present invention is to minimize economicalloss in replacement of the cylinder for preventing X-ray generation byprovision of such cylinder which is removably disposed between theelectrode and the wall of the electron gun.

The present invention will be now described in detail in conjunctionwith the accompanying drawings which show illustrative embodiments ofthe invention and in which:

FIG. 1 shows an elevation in longitudinal cross section of an electronmicroscope embodying the present invention and FIG. 2 shows an elevationin longitudinal cross section of an electron gun of another embodimentof the present invention.

Referring to FIG. 1 there is illustrated an electron device embodyingthe invention which comprises a camera section 1 and a microscopesection 2. The microscope section 2 includes an electron gun 3 and anelectron lens system 4. A cable head 8 is secured to the top of the wall5 of the electron gun by means of screws 7 and 7 through theintermediary of a packing 6. The cable head 8 is provided with a highvoltage insulator 9 which is in turn provided with a cathode 10 and agrid 11. First condenser lens 12 is disposed below the grid 11 andprovided with an anode 13 by means of threaded connection. An exhaustpipe 15 is connected with the wall of the electron device at the sidethereof. A double condenser lens 17, objective 18, intermediate lens 19and a projection lens 20 are mounted in described order from the upperside upon the wall 16 of the electron lens system 4. The doublecondenser lens 17 consists of the first condenser lens 12 and a secondcondenser lens 21. First specimen chamber 22 is formed betwen the secondcondenser lens 21 and the objective 18. The first specimen chamber 22 isprovided with a specimen holder 24 for supporting a specimen 23. Anexhaust pipe 25 is connected to the wall 16 of the device 3 at the sidethereof in communicating relation with the first specimen chamber 22. Aspecimen holder 28 for supporting a specimen 27 can be disposed in asecond specimen chamber 26 formed between the intermediate lens 19 andthe projection lens 20. An exhaust pipe 29 is connected with the wall 16of the device at the side thereof in communicating relation with thesecond specimen chamber 26. Below the projecting lens 20 is arranged anobserving chamber in which is disposed a fluorescent screen 31 at thebottom thereof and which is provided with an observing window 32. Alayer, 33 for preventing 'X-ray generation which is made of a lightmetal material such as aluminium or magnesium is mounted on the innersur-.

face of the walls 5 and 16 of the device in various areas upon whichrecoil or secondary electrons impinge.

Now another embodiment illustrated in FIG. 2 will be described. Likenumerals are used for parts in FIG. 2 which correspond to those shown inFIG. 1 and description is done only for parts which differ from those ofFIG. 1. In place of the layer 33 for preventing X-ray generation whichis mounted on the inner surface of the wall 5 of the electron gun 3, acylinder 14 for preventing X-ray generation made of light metal materialsuch as aluminium or magnesium is inserted in fitting relation betweenthe cable head 8 and the first condenser lens 12 and the electron gunwall 5.

When the microscope section 2 of the embodiment of the present inventionconstructed as mentioned above and shown in FIG. 1 or 2 is highlyevacuated through the evacuation pipes 15, 25 and 29, electrons emittingfrom the cathode travel straight to the anode 13 under the control ofthe grid 11. The electrons passing through the anode 13 are impingedupon the specimen 23 which is supported in the specimen holder 24 bymeans of the double condenser lens 17 comprising the first condenserlens 12 and the second condenser lens 21. The electron beam passingthrough the specimen is enlarged by the objective lens 18, theintermediate lens 19 and the projection lens 20 to impinge upon thefluorescent screen 31. An electron microscope image may be observed onthe screen 31 through the observing window 32.

By removing the specimen holder 24 and disposing the specimen holder 28in the path of electron beam within thesecond specimen chamber 26, anelectron diffraction image of high resolving power of the specimen 27may be observed on the fluorescent screen 31.

Since, in FIG. 1, primary electrons impinge upon the bodies disposed inthe path of the electron beam, for example, the specimen holder 24, theanode'13 and the fluorescent screen 31, primary X-ray is radiated fromthese parts. This primary X-ray is partially absorbed by the layer 33for preventing X-ray generation and the remainder of the primary X-rayis completely absorbed by the walls 5 and 16 of the device. Further,from the bodies disposed in the path of the electron beam are radiatedreflected electrons and secondary electrons to impinge upon the layer 33for preventing X-ray generation. Since the layer 33 for preventing X-raygeneration, however, is made of light metal material such as aluminiumor magnesium, secondary X-ray generation by impingement of the reflectedand secondary electrons may be maintained at very low level.Consequently even if a very small quantity of secondary X-ray istransmitted through the layer 33 for preventing X-ray generation, it maybe completely absorbed by the walls 5 and 16 of the device as energy ofthis secondary X-ray is very low. As a result, outward leakage of theprimary and secondary X-rays can be perfectly prevented. It has beenfound from experiment that when the walls 5 and 16 are in such thicknessthat outward leakage of only primary X-ray may be prevented, the layer33 for preventing X-ray generation of several microns in thickness canprevent perfectly even outward leakage of the secondary X-ray. In otherwords, by using the layer 33 for preventing X-ray generation of severalmicrons in thickness, outward leakage of the secondary X-ray may benecessarily prevented if the walls 5 and 16 of the device have suchthickness that outward leakage of only the primary X-ray can beprevented'Thus the thickness of the walls 5 and 16 may be determineddepending upon outward leakage of the primary Xray which has beenpartially absorbed by the layer 33 for preventing X-ray generation.Consequently the walls 5 and 16 of the device maly be reduced inthickness and manufactured economical y.

The layer 33 for preventing X-ray generation may be formed very easilyby applying a foil of light metal onto the inner surface of the walls 5and 16 or by vacuum spattering or spray of fused metal. The metalutilized in accordance with the invention is selected from the group ofmetals known as the light metals. As indicated in the Concise Chemicaland Technical Dictionary, published by the Chemical Publishing Co.,Inc,, 1962, edited by H. Bennett, the light metals are a group of metalsof specific gravity up to approximately 3; the alkali metals; thealkaline earth metals, e.g., beryllium, magnesium, and aluminum.

Referring to FIG. 2, primary X-ray is generated by) for example, theanode, disposed in the path of electron beam. This primary X-ray ispartially absorbed by the cylinder 14 for preventing X-ray generationand the remainder of the primary X-ray is perfectly absorbed by the wall5 of the device. Besides the primary X-ray, reflected and secondaryelectrons are emitted from the bodies disposed in the path of theelectron beam to impinge upon the cylinder 14' for preventing X-raygeneration. Since the cylinder 14 is made of light metal, such asaluminium or magnesium, generation of secondary X-ray may be suppressedto a minimum. Accordingly even if a very small amount of secondary X-rayis transmitted through the cylinder 14' for preventing X-ray generation,it is absorbed completely by the wall 5 of the electron device. Althoughthe cylinder 14' for preventing X-ray generation may be in thickness ofthe same order as that of the layer 33 for preventing X-ray generationshown in FIG. 1 is suflicient to perfectly prevent outward leakage ofthe primary and secondary X-rays, the layer 14' of 1 to 2 mm. thicknessis preferable from the view point of mechanical strength.

The cylinder 14 for preventing X-ray generation having thickness to thisextent can be easily made by press Work and be process into a definiteshape. The inner surface thereof can be finished easily and precisely bybuffing, electro-polishing or chemical polishing without anyrestriction. Moreover, light metal is relatively soft so that thepin-holes may be easily stopped by the above described polishing works.Thus the inner surface of the cylinder 14' for preventing X-raygeneration can be finished into substantial speculum without anyirregularities and pinholes. Consequently, discharge which may beproduced between the grid 11 and the wall 5 of the electron device canbe minimized so that disturbance of accelerated electron beam and damageof the cylinder 14' for preventing X-ray generation due to the dischargemay be minimized.

Even if the cylinder 14': should fail in any accident, heavy damage asexperienced by the prior art devices can be avoided as this cylinder 14'for preventing Xray generation is moderate in price. This cylinder 14'may be replaced easily by removing the cable head 8 from the wall 5 ofthe electron device by releasing'the screws 7 and 7.

Although heretofore the present invention has been described inconnection with an electron microscope having three-lens system, it hasbeen done merely for illustrative purpose and it is to be understoodthat the invention is applicable to various devices utilizing electronbeam, as for example, electron beam machining apparatus and X-raymicroanalyser.

As described above in detail, the present invention has many practicaladvantages that outward leakage of primary and secondary X-rays areperfectly prevented by provision of a layer for preventing X-raygeneration without thickening the wall of the electron device and thelayer for preventing X-ray generation, occurrence of discharge isminimized by provision of a removable cylinder for preventing X-raygeneration and economical loss can be reduced in the case of replacementof the cylinder for preventing X-ray generation.

What is claimed is:

1. An electron microscope comprising an electron gun section and amicroscope section for guiding an electron beam emitted by said electrongun section; said electron gun section being provided with electrodesfor emitting said electron beam and guiding said electron beam into saidmicroscope section and a wall surrounding said electrodes; saidmicroscope section being provided with first lens means to focus saidelectron beam, second lens means to expand said focused electron beam, aspecimen chamber formed between said first and second lens means, anobservation chamber for observing a specimen disposed in said specimenchamber and for guiding said expanded electron beam therethrough and afluorescent plate disposed in said observation chamber and adapted toreceive said expanded electron beam, the wall of said electron gunsection and the Wall defining said specimen chamber and said observationchamber being provided on the inner surfaces thereof with a metal layerselected from a group consisting of the light metals to effectivelyprevent X-ray generation.

2. An electron microscope according to claim 1 wherein said layers forpreventing X-ray generation are each composed of aluminum.

3. An electron microscope according to claim 1 wherein the metal layeron the wall surrounding said electrodes is in the form of a cylinder forpreventing X-ray generation provided between said wall surrounding theelectrodes and said electrodes in said electron gun section.

4. An electron microscope according to claim 3 wherein said cylinder forpreventing X-ray generation is made of aluminum.

5. An electron microscope according to claim 3 Wherein said cylinder forpreventing X-ray generation is provided in removable fashion.

6. An electron microscope according to claim 5 wherein said removablecylinder for preventing X-ray generation is made of aluminum.

7. An electron microscope according to claim 5 wherein said removablecylinder for preventing X-ray generation is made of a metal from thegroup essentially consisting of aluminum and magnesium.

8. An electron device comprising an electron gun section and amicroscope section for guiding an electron beam emitted by said electrongun section; said electron gun section being provided with electrodesfor emitting said electron beam and guiding said electron beam into saidmicroscope section, and a wall surrounding said electrodes; saidmicroscope section being provided with lens means to focus said electronbeam, a specimen chamber for guiding said focused electron beam, and aspecimen disposed in said specimen chamber to receive said focusedelectron beam; and the wall of said electron gun section and the walldefining said specimen chamber being provided on the inner surfacesthereof with a metal material layer selected from a group consisting ofthe light metals to prevent X-ray generation.

9. An electron device according to claim 8 wherein the layer on the wallsurrounding said electrodes is in the form of a cylinder for preventingX-ray generation provided between said wall surrounding the electrodesand said electrodes in said electron gun section.

References Cited UNITED STATES PATENTS 2,418,321 4/1947 Smith 250-4953,018,398 1/1'962 Atlee 31359 3,087,061 4/1963 Dukes et a1 2508'3.3

OTHER REFERENCES An Experimental Electron Microscope for 400 Kilovoltsby A. C. van Dorsten et al. from Philips Technical Review, vol. 9, Nov.7, 1947, pp. 193-201, pp. 199- 201 relied on.

RALPH G. NILSON, Primary Examiner. W. F. LINDQUIST, Assistant Examiner.

1. AN ELECTRON MICROSCOPE COMPRISING AN ELECTRON GUN SECTION AND AMICROSCOPE SECTION FOR GUIDING AN ELECTRON BEAM EMITTED BY SAID ELECTRONGUN SECTION; SAID ELECTRON GUN SECTION BEING PROVIDED WITH ELECTRODESFOR EMITTING SAID ELECTRON BEAM AND GUIDING SAID ELECTRON BEAM INTO SAIDMICROSCOPE SECTION AND A WALL SURROUNDING SAID ELECTRODES; SAIDMICROSCOPE SECTION BEING PROVIDED WITH FIRST LENS MEANS TO FOCUS SAIDELECTRON BEAM, SECOND LENS MEANS TO EXPAND SAID FOCUSED ELECTRON BEAM, ASPECIMEN CHAMBER FORMED BETWEEN SAID FIRST AND SECOND LENS MEANS, ANOBERVATION CHAMBER FOR OBSERVING A SPECIMEN DISPOSED IN SAID SPECIMENCHAMBER AND FOR GUIDING SAID EXPANDED ELECTRON BEAM THERETHROUGH AND AFLUORESCENT PLATE DISPOSED IN SAID OBSERVATION CHAMBER AND ADAPTED TORECEIVE SAID EXPANDED ELECTRON BEAM, THE WALL OF SAID ELECTRON GUNSECTION AND THE WALL DEFINING SAID SPECIMEN CHAMBER AND SAID OBSERVATIONCHAMBER BEING PROVIDED ON THE INNER SURFACE THEREOF WITH A METAL LAYERSELECTED FROM A GROUP CONSISTING OF THE LIGHT METALS TO EFFECTIVELYPREVENT X-RAY GENERATION.